Biodiesel and high-value products from microalgae are researched in many countries. Compared to first generation biofuel crops, advantages of microalgae do not only lead to economic benefits but also to better environmental outcomes. For instance, growth rate and productivity of microalgae are higher than other feedstocks from plant crops. In addition, microalgae grow in a wide range of environmental conditions such as fresh, brackish, saline and even waste water and do not need to compete for arable land or biodiverse landscapes. Microalgae absorb CO2 and sunlight from the atmosphere and convert these into chemical energy and biomass. Thus, the removal of CO2 from the atmosphere plays a very important role in global warming mitigation, as the produced biofuel would replace an equivalent amount of fossil fuel. Based on their high protein contents and rapid growth rates, microalgae are also highly sought after for their potential as a high-protein containing feedstock for animal feed and human consumption. However, despite the promising characteristics of microalgae as a feedstock for feed and fuel, their stable cultivation is still difficult and expensive, as mono-species microalgae can often get contaminated with other algae and grazers.
To address this issue I hypothesized that indigenous strains have a highly adaptive capacity to local environments and climatic conditions and therefore may provide good growth rates in the same geographic and climatic locations where they have been collected from. Collecting from fluctuating environments, e.g. rock pools, that undergo extreme environmental changes, may also increase the chances of isolating strains with a high lipid accumulation capability, a trait believed to increase microalgae’s survival rate. “Local microalgae from extreme environments have higher survival and lipid accumulation capability” was used as a working hypothesis with the main aim to isolate microalgal strains that are useful for large-scale cultivation. Thus, this thesis, apart from the biological question, also has a bioprocess engineering focus and the requirement to develop suitable microalgal strains for large-scale production.
Although it is often stated that high lipid accumulation capacity leads to higher survival rates under adverse conditions, to our knowledge this basic assumption has not been proven experimentally. Therefore, microalgae with the ability to accumulate lipids were cultured under defined laboratory conditions and divided into two populations (with and without a certain level of cellular triacylglycerides) using flow cytometry. Subsequent survival tests confirmed that microalgal cells from the oleaginous population had higher survival rates under nutrient starvation conditions at all timepoints tested than the population with lower lipid contents. To follow up with this question in natural habitats, environmental factors were monitored at three field sites, including a tidal brackish river, a mangrove forest and several beach rock pools. The effects on lipid accumulation in microalgae as well as the entire eukaryotic micro-community structure were profiled by 18S pyrotag amplicon sequencing. The results showed that aquatic microorganism diversity was influenced by habitat conditions, such as water flow and nutrient supplies. Besides, lipid accumulation in microalgae was higher in adverse environmental conditions, further supporting the initial hypothesis.
To advance the bioprospecting aspect of the thesis, indigenous microalgal strains from local Australian waters that undergo environmental changes were therefore isolated and evaluated as potential feedstock for the production of biodiesel, protein-rich animal feed and high value products, such as omega-3 fatty acids. Specifically, identification of microalgae from indigenous habitats was brought into context with screening for their potential capacity of producing significant amounts of oil and protein. Most samples were collected from freshwater and seawater in Queensland and the Northern Territory, the intended destination of large-scale microalgae farms for fuel and cattle feed, respectively.
After an elaborate screening process, three potential strains Chlorella sp. BR2, Chlorella sp. NT8a and Scenedesmus dimorphus NT8e that produced high lipid and protein contents were chosen for optimization of culture conditions. Well controlled conditions of temperature, pH and nutrients during the cultivation period led to different lipid and protein productivities. There was no significant influence of pH and nutrients on the biochemical productivity. However, temperature was the factor that affected biochemical compound production the most. Chlorella sp. NT8a and Chlorella sp. BR2 produced high lipid and protein contents, reaching 30% (dry weight) and 50% - 60% (dry weight), respectively, at 30°C. Scenedesmus dimorphus NT8e produced lipids reaching 20% (dry weight) and protein reaching 45% (dry weight) at 35°C. In addition, Scenedesmus dimorphus NT8e has good capacities of adaptation and the ability to settle over night, an important feature to reduce harvesting costs. Productivity of the strain was further improved when cultured in outdoor closed bioreactors and open pond cultivation systems. Scenedesmus sp. NT8e has then been adopted for large-scale cultivation at the Algae Energy Farm in Pinjarra Hills, Australia, where biofuel and animal feed can be produced simultaneously. Based on the results, this strain is recommended for large-scale algae energy farms that can efficiently produce biofuel and animal feed at cattle farms in the Northern Territory of Australia.